glow plug on time?

[Duster 5.9]

what shuts these off. the relay? the caddy i'm working on they don't shut off. i actually took the cover off relay and watched it make connection with  all glow pluggs unplugged. it doesn't shut off. 3 of the 4 pluggs in the car were burnt right off. and what temp sending unit is the one for this. front rad hose? small one in head? or the one for the heater hose?
again what tells them to shut off?

[Duster 5.9]

must be the relay. i installed the one from my daily driver and the light goes out after 15 sec est. and the relay reopens after another 15-20 sec. installed old relay and it stayed on. also tryed one from a parts car i have and that one stayes on to but that car its nothing proven to me, must be bad too. still want to know about the temp wires though and anymore input on the glow pluggs.

[zukgod1]

There is a temp sensor on the head.
If it's unplugged it will act like you are describing.